[clang] [llvm] [DebugInfo] Emit DW_AT_bit_size for _BitInt types and others (PR #164372)

[David Stenberg via llvm-commits]

dstenb wrote:

Thanks for picking this up, @OCHyams, and sorry for some delay in reviewing this!

I see that you discussed omitting `DW_AT_data_bit_offset` of value 0. I think that we need to amend the patch so that it emits that attribute for non byte size-multiple types on big-endian targets. I think I may have only experimented with x86 when I made that initial patch (I see that there is a commented out line adding that attribute in my patch that I don't remember the history for).

I have confirmed this by trying out this patch on our downstream target with GDB, and I think it can be seen e.g. with the following MIPS example:

```
struct { uint16_t a : 1; uint16_t b : 15; } bf = { .b = 0x3fffU };
unsigned _BitInt(15) bi = 0x3fffU;
```

Memory-wise the two globals are the same, with `bf.a` "emulating" the padding bit in the stored `bi` value. The `bf.b` field gets a `DW_AT_data_bit_offset` of 1, so I think we need to do the same for `bi`.

```
0x0000003c:     DW_TAG_member
                  DW_AT_name    ("b")
                  DW_AT_type    (0x00000047 "uint16_t")
                  DW_AT_decl_file       ("bif.c")
                  DW_AT_decl_line       (3)
                  DW_AT_bit_size        (0x0f)
                  DW_AT_data_bit_offset (0x01)
```

https://github.com/llvm/llvm-project/pull/164372